Recognition of molecular patterns characteristic of microbes or altered-self leads to immune activation in multicellular eukaryotes. In Arabidopsis thaliana, the leucine-rich-repeat receptor kinases FLAGELLIN-SENSING2 (FLS2) and EF-TU RECEPTOR (EFR) recognize bacterial flagellin and elongation factor EF-Tu (and their elicitor-active epitopes flg22 and elf18), respectively. Likewise, PEP1 RECEP-TOR1 (PEPR1) and PEPR2 recognize the elicitor-active Pep epitopes conserved in Arabidopsis ELICITOR PEPTIDE PRECURSORs (PROPEPs). Here we reveal that loss of ETHYLENE-INSENSITIVE2 (EIN2), a master signaling regulator of the phytohormone ethylene (ET), lowers sensitivity to both elf18 and flg22 in different defense-related outputs. Remarkably, in contrast to a large decrease in FLS2 expression, EFR expression and receptor accumulation remain unaffected in ein2 plants. Genome-wide transcriptome profiling has uncovered an inventory of EIN2-dependent and EFR-regulated genes. This dataset highlights important aspects of how ET modulates EFR-triggered immunity: the potentiation of salicylate-based immunity and the repression of a jasmonate-related branch. EFR requires ET signaling components for PROPEP2 activation but not for PROPEP3 activation, pointing to both ET-dependent and -independent engagement of the PEPR pathway during EFR-triggered immunity. Moreover, PEPR activation compensates the ein2 defects for a subset of EFR-regulated genes. Accordingly, ein2 pepr1 pepr2 plants exhibit additive defects in EFR-triggered antibacterial immunity, compared with ein2 or pepr1 pepr2 plants. Our findings suggest that the PEPR pathway not only mediates ET signaling but also compensates for its absence in enhancing plant immunity.microbe-associated molecular pattern | danger-associated molecular pattern | disease resistance | plant hormone | defense-related genes
Recognition of microbe-associated molecular patterns (MAMPs) leads to the generation of MAMP-triggered immunity (MTI), which restricts the invasion and propagation of potentially infectious microbes. It has been described that the perception of different bacterial and fungal MAMPs causes the repression of flavonoid induction upon light stress or sucrose application. However, the functional significance of this MTI-associated signaling output remains unknown. In Arabidopsis (Arabidopsis thaliana), FLAGELLIN-SENSING2 (FLS2) and EF-TU RECEPTOR act as the pattern recognition receptors for the bacterial MAMP epitopes flg22 (of flagellin) and elf18 (of elongation factor [EF]-Tu), respectively. Here, we reveal that reactive oxygen species spiking and callose deposition are dispensable for the repression of flavonoid accumulation by both pattern recognition receptors. Importantly, FLS2-triggered activation of PATHOGENESIS-RELATED (PR) genes and bacterial basal defenses are enhanced in transparent testa4 plants that are devoid of flavonoids, providing evidence for a functional contribution of flavonoid repression to MTI. Moreover, we identify nine small molecules, of which eight are structurally unrelated, that derepress flavonoid accumulation in the presence of flg22. These compounds allowed us to dissect the FLS2 pathway. Remarkably, one of the identified compounds uncouples flavonoid repression and PR gene activation from the activation of reactive oxygen species, mitogen-activated protein kinases, and callose deposition, corroborating a close link between the former two outputs. Together, our data imply a model in which MAMP-induced repression of flavonoid accumulation serves a role in removing the inherent inhibitory action of flavonoids on an MTI signaling branch.
-Ascidians, or sea squirts, are lower chordates, and share basic gene repertoires and many characteristics, both developmental and physiological, with vertebrates. Therefore, decoding cis -regulatory systems in ascidians will contribute toward elucidating the genetic regulatory systems underlying the developmental and physiological processes of vertebrates. cis -Regulatory DNAs can also be used for tissuespecific genetic manipulation, a powerful tool for studying ascidian development and physiology. Because the ascidian genome is compact compared with vertebrate genomes, both intergenic regions and introns are relatively small in ascidians. Short upstream intergenic regions contain a complete set of cis -regulatory elements for spatially regulated expression of a majority of ascidian genes. These features of the ascidian genome are a great advantage in identifying cis -regulatory sequences and in analyzing their functions. Function of cis -regulatory DNAs has been analyzed for a number of tissue-specific and developmentally regulated genes of ascidians by introducing promoter-reporter fusion constructs into ascidian embryos. The availability of the whole genome sequences of the two Ciona species, Ciona intestinalis and Ciona savignyi , facilitates comparative genomics approaches to identify cis -regulatory DNAs. Recent studies demonstrate that computational methods can help identify cis-regulatory elements in the ascidian genome. This review presents a comprehensive list of ascidian genes whose cis -regulatory regions have been subjected to functional analysis, and highlights the recent advances in bioinformatics and comparative genomics approaches to cis -regulatory systems in ascidians.
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